1. Field of the Invention
The present invention relates to a switching power supply of a pseudo-resonance type in which a partial resonance (for example, sinusoidal variation) occurs.
2. Related Art
The switching power supply has a decreased switching frequency, as a load thereof decreases. Accordingly, it suffers a later-described ringing voltage (for example, X1 in FIG. 2) developed therein, as the load is decreased light, whereby the oscillation and output become unstable.
The present invention was made to solve this problem, and has as an object to provide a switching power supply capable of providing stable oscillation and output even with a light load.
To achieve the above-noted object, according to a first aspect the present invention, there is provided a switching power supply having a switching element, which switches a DC current supplied to a transformer from a DC power supply so as to generate a voltage at the secondary side of the transformer and a capacitor determining the off time of the switching element, wherein when a ringing voltage is generated under a low load, at a time when the voltage is lower than a lower valley part of this voltage, the charging voltage of the capacitor is raised and the off time is extended.
According to a second aspect of the present invention, there is provided a switching power supply having a switching element, which switches a DC current supplied to a transformer from a DC power supply so as to generate a voltage at the secondary side of the transformer and a capacitor determining the off time of the switching element, this switching power supply further having a bottom detection circuit, which determines whether or not a bottom has been reached based on the voltage of the capacitor and a voltage applied to an over-current protection terminal, and a charging voltage switching circuit, which, in a case in which the bottom detection circuit judges that a bottom has been exceeded, switches the voltage applied to the capacitor, and which raises the charging voltage of the capacitor when the bottom is exceeded, so as to extend the off time.
According to a third aspect of the present invention, there is provided a variation on the switching power supply of the second aspect, wherein the bottom detection circuit has a first time period detection circuit, which detects a first time period after which the voltage of the capacitor drops below a first reference value, and a second time period detection circuit, which detects a second time period after which the voltage of the capacitor drops below a second reference value, the switching power supply further having a switching signal generation circuit, which generates a switching signal for a period during which bottom is set to on, outputting this signal to the charging voltage switching circuit.